Electric shock therapy of ventricular fibrillation is an emergency procedure which is literally life saving. However, even a short delay of a few seconds may result in permanent severe damage or death of the patient. In order to minimize the time delay between onset of fibrillation and appropriate therapy, it would be advantageous to have a $ "thinking defibrillator" which would automatically measure the thoracic impedance and deliver the correct dose of electric energy and current to defibrillate the ventricles. Development of such a device depends upon careful quantitation of the strength of shock required to defibrillate under various conditions for patients of various body size. It is the purpose of this investigation to determine the effects upon defibrillation requirements for various electric wave-forms in the presence of drugs and altered metabolic state of the heart. Also impedance characteristics of the thorax must be quantitated in order that design information will be available for development of such a "thinking defibrillator".